1. Field of the Invention
The present invention relates to a light emitting diode (LED) unit, more specifically to an LED unit configured to have high luminous efficiency, good heat release characteristics, durability and a simple and easily assembled structure.
2. Description of Related Art
In recent years, there has been a notable trend toward development of increasingly high output LEDs for use in various appliances, such as OA instruments, indicator lamps for vehicle instruments, indicator lamps for traffic signals, front lights and the like. However, increasing the amount of light emitted from an LED unit also increases the heat generated within the LED chip, leading to a lowering in luminous efficiency of the LED unit. Moreover, an LED unit including a resinous reflector to enhance reflection efficiency of light emitted from an LED chip has low reflection efficiency because the reflector is deteriorated due to the significant rise in temperature within the LED unit. Furthermore, if an LED chip is mounted on a metallic base, there is a problem that the LED chip may become separated from the base. Therefore, proposals have been made for an LED unit which can provide improvements in heat release property, and can prevent heat-induced deterioration of the reflector and heat-induced separation of the LED chip from the base (for reference, see Japanese Patent Laid-Open No. 2005-136137, page 4, FIG. 1).
FIGS. 5 and 6 illustrate an LED unit disclosed in Japanese Patent Laid-Open No. 2005-136137.
The LED unit includes a base 1, a case 2 attached to the base 1, an LED chip 3 supported on the base 1, and a lens section 4 which is disposed above the LED chip 3 and attached to the case 2, as shown in FIGS. 5 and 6.
The case includes a central hole 2a in which the base 1 is inserted and a concave portion 2b which includes an inclined surface continuing to the central hole 2a, and has a rectangular outer shape. A surface treatment such as silver plating to reflect light emitted from the LED chip 3 is applied to a surface of the concave portion 2b. A first electrode 5 and a second electrode 6 each of which is formed by a conductive film are disposed on an upper surface of the base 1. The first and second electrodes 5 and 6 are adhered to the base 1 by an insulative adhesive. An end portion 7a of a first lead 7 is connected to the first electrode 5 and an end portion 8a of a second lead 8 is connected to the second electrode 6 (see FIG. 6).
The first and second leads 7 and 8 are adhered to the case 2 by an insulative adhesive such as low-melting point glass.
The LED chip 3 is mounted on the first electrode 5 and adhered to the first electrode 5 by a conductive adhesive such as a silver paste, thereby allowing a lower surface electrode of the LED chip 3 to be electrically connected through the first electrode 5 to the first lead 7. On the other hand, an upper surface electrode of the LED chip 3 is electrically connected through a bonding wire 9 to the second electrode 6.
As described above, in the LED unit as shown in FIGS. 5 and 6, the LED chip 3 is disposed on the first electrode 5 and light emitted from the LED chip 3 is reflected on the inclined surface of the concave portion 2b. 
However, in the above-mentioned LED unit, because the LED chip 3 is disposed on the first electrode 5, the problems arise that it is difficult to release the heat of the LED chip 3 to the exterior through the base 1, and that the surface of the concave portion 2b to which the silver plating treatment is applied receives a relatively large amount of direct heat from the LED chip and may become oxidized or sulfurized, leading to a reduction in reflection efficiency of the surface.